1. Field of the Invention
The present invention pertains to a suction lifter for a material handling device for workpieces, comprising suction surfaces and at least one pneumatic cylinder with a suction cup arranged at its end.
2. Description of the Prior Art
Arrangements are known in which workpieces are sucked up and lifted with such suction lifters for subsequent transportation in manufacture. This is common practice, e.g., in sheet metal fabrication, where a cut-out piece of sheet metal is lifted out of the basic sheet metal for further processing using the suction lifter. Subsequently, either the basic sheet metal or the piece cut out is removed. After positioning, the workpiece is again released by the suction lifter for further processing. It is possible, for example, to arrange a series of such suction lifters in parallel in order to pick up pieces punched out from a large sheet metal plate. It is also known that using appropriate controls, it is possible to pick up only some of the pieces punched out or to activate only some of the suction lifters present.
In a prior-art suction lifter of this class, in which the working cylinder is designed as a constant-pressure cylinder, the piston rod passes through both end faces of the cylinder, and the piston rod, which is of hollow design, carries the suction cup at one of its ends. A connection for the vacuum, which connection is controlled via a directional control valve, is provided at the other end of the piston rod. The stroke of the suction cup is also controlled via a directional control valve, which connects the pressure source or the vent hole with the first working chamber or with the second working chamber.
The control system for such a suction lifter is relatively expensive, because there are two directional control valves operating independently of one another for each suction lifter, and the directional control valves must be finely coordinated with one another and with other valves of the suction lifter. This makes the entire control device complicated and requires a number of additional position transducers.